An engine usually runs with all of its cylinders. However, when this engine works at low or partial loads, the engine efficiency is degraded by the increase in the contribution of frictions and of throttling in the case of gasoline type engines.
It has been proposed operating only part of the cylinders of this engine, thus making the remaining part inactive. Fuel injection is therefore stopped only in the cylinders to be deactivated. This allows the reduction of fuel consumption to be favoured by injecting the fuel required only for the cylinders necessary for production of the energy needed to operate the engine at low or partial loads.
Although this engine running type is satisfactory, it however involves some quite significant drawbacks.
In fact, since the lift laws of the exhaust and intake valves remain unchanged, the different phases known as expansion and compression of the inactive cylinder(s) will lead to running problems.
Thus, as it is well known, during the usual expansion phase of the cylinder of an engine, the combustion chamber of this cylinder contains a compressed fuel mixture that burns away while carrying out expansion of the volume present in this chamber. During this phase, the exhaust and intake valves are closed and the piston is driven in a motion towards its bottom dead center under the effect of the expansion of the burnt gas by transmitting the energy thereof to the crankshaft it is connected to.
When cutting off fuel supply to the cylinder to be made inactive, no fuel mixture occurs in the combustion chamber and only a volume of compressed air is present. During the phase of this cylinder which corresponds to its expansion phase, the piston is not subjected to a force resulting from the expansion of the burnt gases but it only stretches a volume of air compressed by the previous engine running phase. This stretching generates cooling of the air contained in the cylinder and this temperature drop is transmitted to the oil film usually present on the cylinder wall. The oil film cools down and loses its lubricating qualities, notably as regards the viscosity thereof, which leads to bad lubrication between the piston and the cylinder, which may cause sticking between these two parts.
This cooling is still more marked during the exhaust phase of the inactive cylinder, during motion of the piston from the bottom to the top of the cylinder. During this motion, the cold expanded air is pushed by the piston towards the exhaust valve and travels the surface of the oil film. This increases cooling of the oil film with a greater loss of the lubricating qualities thereof. Still more disadvantageously, this air leaving the chamber is sent to the exhaust channel by feeding a significant amount of oxygen into the hot gases from the combustion of the active cylinders. This oxygen supply is not only harmful for the reliability of the exhaust gas fuel/air ratio measurement, in case a depollution means is used, it also involves an exhaust gas afterburning risk if this gas contains a sufficient residual amount of fuel.
Furthermore, during the expansion phase of the inactive cylinder, the piston that slides in this cylinder is no longer a piston generating energy to be transmitted to the crankshaft. In fact, the combustion chamber contains no compressed fuel mixture that expands upon combustion, but only compressed air. This piston therefore acts as an energy absorber. In fact, since no combustion occurs in this cylinder, the piston is only driven, in a motion from the top to the bottom of this cylinder, by the crankshaft that is subjected to a rotating motion produced by the active cylinder(s). These cylinders therefore have to generate an additional energy to drive the piston of the deactivated cylinder. This additional energy is all the higher as the piston lubrication conditions in the inactive cylinder are degraded by the expansion of the compressed air.
The present invention aims to overcome the aforementioned drawbacks by means of a simple and economical method allowing to reduce even more the fuel consumption and to minimize the energy loss due to the inactive cylinder.